Our Research

Our lab focuses on a variety of topics that involve marine ecosystems, however, we are particularly interested in marine diseases. A substantial amount of our research focuses on the microbe-host and microbe-microbe interactions that affect host health. This has lead us to investigate coral disease outbreaks well as the invertebrate pathogen Vibrio coralliilyticus. Furthermore, our evolving work on coral probiotics is focused on the development of new treatments for coral diseases as well as understanding how some of these effective probiotics actually benefit their coral hosts. Our lab predominantly takes microbial and genetics-based approaches to understanding disease, however, we are also interested in a wide variety of topics.

Stony coral tissue loss disease (SCTLD)

Since 2014, a devastating coral disease has been spreading throughout the Florida Reef Tract. This disease, stony coral tissue loss disease (SCTLD), causes the progressive death of the coral animal. Alarmingly, SCTLD affects at least 20 different Caribbean coral species, some of which are classified as endangered. Unfortunately, in 2018, SCTLD has started to spread throughout the Caribbean.

The main cause of SCTLD is unknown but pathogenic bacteria are involved with the disease process. Various groups of bacteria have been identified to be associated with diseased corals or their surrounding sediment, however, their current role in SCTLD is unknown. What is known is that this disease can apparently spread through the water column or direct contact with infected fragments, suggesting that the primary agent responsible for SCTLD is infectious. More research is needed, but what is know is that the SCTLD front appeared to move at approximately 100 m per day as it spread through the Florida Reef Tract.

A recent discovery was that some corals with SCTLD may be colonized by the pathogenic bacterium Vibrio coralliilyticus. In collaboration with mAbDx Inc. and Dr. Claudia Häse at Oregon State University, an immunoassay was developed to detect the toxic protein produced by V. coralliilyticus. Using this immunoassay and a digital droplet PCR assay developed by Dr. Julie Meyer at the University of Florida, a correlation was discovered between the apparent presence of this bacterium and more virulent SCTLD was discovered. Although V. coralliilyticus is not believed to be a cause of SCTLD, this bacterium may be responsible for coinfections exacerbating the effects of SCTLD. Read the full story here.

Coral Probiotics for SCTLD

Take a look at our publication on the first probiotic treatment effective against SCTLD! (link)

Currently, there are a limited number of treatments for SCTLD (or any coral disease), which have included culling colonies to applying epoxy mixed with powdered bleach. However, the most effective treatment so far has been the application of the antibiotic amoxicillin mixed with a special paste to disease corals. Unfortunately, the amoxicillin paste can only treat active lesions and not confer lasting protection (e.g. a vaccine). Additionally, there is a risk of selecting for antibiotic-resistant microorganisms from its use in the environment, while also rendering the treatment ineffective.

An alternative to this is the use of beneficial microorganisms, probiotics. This is based on the concept that the microorganisms that colonize healthy corals can impart some benefit to the host organism (the coral animal). Some of these beneficial microbes are thought to protect the coral from pathogenic microbes, sometimes through the production of antibacterial or antibiotic compounds. However, some corals may have a “better” microflora (the collection of microorganisms living on it) than other corals, so they could be more resistant to SCTLD than other corals. Therefore, the Ushijima lab has focused on utilizing these probiotic microorganisms (mostly bacteria) to treat and protect Caribbean corals in hopes of combating SCTLD.

Dr. Ushijima started his investigations into coral probiotics when has was a graduate student in Hawaiʻi working on Pacific coral disease. Since then, he was one of the founders of the Coral Health and Marine Probiotics (CHAMP) Laboratory at the Smithsonian Marine Station in Florida, which focused on developing probiotics to treat SCTLD.

Dr. Ushijima is continuing to work with the CHAMP Lab and have begun field testing coral probiotics on SCTLD-affected coral colonies in Florida. To actually deploy probiotics onto corals, a few novel tools have been developed.

Molecular Pathogenesis of Vibrio coralliilyticus

The bacterium Vibrio coralliilyticus is a pathogen implicated in infections of a variety of invertebrates, which includes species of oysters, clams, scallops, urchins, and corals. However, unlike Vibrio spp. that infect humans, comparatively little is known about how V. coralliilyticus causes disease and the associated molecular mechanisms. Why this is important is because investigating how this pathogen causes disease will allow us to develop improved and specific diagnostic or mitigation tools. A great example is the immunoassays (described above) that detect the toxic metalloprotease produced by virulent strains.

Influence of environmental factors on virulence

For corals, the initial isolates of V. coralliilyticus were found to induce bleaching in the coral Pocillopora damicornis, but when water temperatures increased the infection resulted in tissue loss. This drastic response to water temperature suggests that the changing environmental conditions may favor increased virulence and outbreaks like pathogens like V. coralliilyticus. It was discovered that expression of the regulatory gene, toxR, can respond to temperature, with increased expression occurring at elevated growth temperatures. Interestingly, for V. coralliilyticus strains that induce tissue loss regardless of water temperature, toxR expression is consistent. In contrast, for strains that have their virulence stringently regulated by water temperature, toxR expression follows the expected response to temperature.

Additional studies discovered that one of the genes hypothesized to be regulated by ToxR, the predicted outermembrane protein OmpU, is essential for infections of rice coral and Pacific oyster larvae. However, there are various other virulence gene regulators, the environmental signals that influence their expression, and the specific virulence factors that are still need to be characterized.

Chemotaxis and coral infection

Our lab is also interested in the chemotaxis and motility of V. coralliilyticus. Research from other groups suggested that this pathogen chemotaxes towards corals via chemical signals released by the host holobiont. However, it is likely much more complicated than that. Using mutants of V. coralliilyticus, it was discovered that some non-chemotactic strains were avirulent while others were hypervirulent. Subsequent analysis discovered that the swimming pattern influenced how virulent each strain was. Therebefore, our lab would like to further understand how this motility and chemotaxis, and the regulation of these activities, play into the transmission of V. coralliilyticus.

The Type VI Secretion System

In addition to a suite of potential virulence factors, Vibrio coralliilyticus possesses countermeasures to outcompete with the protective microflora of potential hosts. One of these is the Type VI Secretion System (T6SS), a contact-dependent antibacterial system. This system was originally discovered in the human pathogen Vibrio cholerae, with similar systems identified in various other pathogens. During our initial investigation, we discovered that V. coralliilyticus could kill off V. cholerae using its own T6SS. Our lab is interested into how the V. coralliilyticus system works and what are the toxic effector molecules associated with this system.